You two are just kids, you should respect her father's wishes absolutely. Don't take his rejection too hard, because it might not have anything to do with you. Something, it could be in the way she has been acting, has caused him to feel the need to assert control over your ex's dating habits. Honestly, he may just not trust you. More than likely, he feels the need to protect her which is his job and at this age she definitely needs her father doing this in her life.
However, if you want to try again to make it work- you have to make it right with Dad. You need to talk to him, man to man and let him know how you feel, what your intentions are, and let him know that you want to fix the relationship between him and you. I'd opt to give him more control over the situation by telling him he can set the rules and boundaries of the relationship. Then follow them. Give him the opportunity to learn to trust you and get to know you.
If he says no, you got to take it because going behind his back will only make things worse. If you two still want to pursue love after that, you both have to go the slow path of becoming people that he trusts as single individuals. Then, maybe in a few years when you're older he'll feel differently about it.
It's a lot of work, yeah, but love always is. It's not just the feeling you find yourself engulfed by. The odds are always going to be against you, more now than ever with the fluidity of marriage these days.(And I'm not trying to assume you guys are close to the marriage part yet, just trying to give you some perspective) 52% of marriages end in divorce and there are a lot of ongoing, loveless marriages. I'm not saying this to discourage you- but if you really do love her, no matter what the current situation comes by, it will include self control, selflessness, patience, and perseverance.